Mustapha Mamboleo, a local official at Idjwi island in the lake, said 150 passengers were aboard the motorised boat traveling from Mugote village on Idjwi island to the eastern city of Goma and 80 of them survived.
Mathieu Alimasi Malumbi, the transport minister for South Kivu province, confirmed the incident.
"I can confirm that there was a capsizing of a motorboat with more than 100 passengers on board, many of whom were not on the manifest," the minister told Reuters via telephone.
An investigation was launched in addition to the search for survivors, he said.
Deadly boat accidents are common in waters of Congo, where vessels are frequently loaded well beyond their capacity.
